Engineering topics

Larimer, John W.

Publications and source records attributed to Larimer, John W..

6 records

NASA NTRS1994

A Comparison of Metallographic Cooling Rate Methods Used in Meteorites

The primary objective of this study was to test the postulate that cooling rates acquired from metal grains in chondrites are consistent with those from iron meteorites. Both types of metal occur in some Group IAB meteorites, which are mixtures of massive metal with well-developed Widmanstatten structures and chondritic inclusions with dispersed metal grains. The grains have textures and compositions similar to chondritic metal, including negligible P. The meteorites studied show little or no sign of shock reheating and textural evidence indicates that silicates and metal were mixed before Widmanstatten patterns formed during cooling. Cooling rates were obtained by comparing measured to modeled taenite grain or lamellae dimensions and central Ni contents. Modeling entails solving diffusion equations using experimental diffusion coefficients, phase relations, and bulk or local Ni and P contents, taking into account geometry, undercooling, and impingement. There is one set of parameters for grains and another, quite different set for Widmanstatten lamellae, including a factor of 30 difference in diffusion coefficients. Yet cooling rates obtained from Widmanstatten structures and metal grains in chondritic inclusions of the same meteorite are consistent; uncertainties in the best data are +/- 10 K/Ma, equivalent to a factor of 1 +/- 0.25. This agreement implies that the data and models are correct or contain fortuitously offsetting errors, which is quite unlikely. Cooling rates range from 40 K/Ma to 70 K/Ma in IAB meteorites that contain both grains and Widmanstatten structures. Rates based on grains in Ni-poor and Ni-rich meteorites lacking Widmansatten patterns expand the range from 30 K/Ma to perhaps 200 K/Ma. Cooling rates correlate with Ni content; Ni-poor meteorites have slower rates than Ni-rich ones. Evidently, IAB meteorites were radially distributed over greater than 30km in a body with a radius less than 50km. A comparison of the available Ar ages with cooling times inferred from the cooling rates suggests that the parent body cooled more slowly after the metallographic cooling rates were established.

Herpfer, Marc A.↗

NASA NTRS1988

The cosmochemical classification of the elements

The present cosmochemical study divides the elements into four groups: refractory, moderately volatile, highly volatile, and siderophile, with the former three respectively condensing at temperatures higher than 1300-1400 K, between 670 and 1300 K, below 670 K. In the case of siderophiles, the essential condition is their being more easily reduced to metal than Fe. The siderophiles tend to behave as a cohesive group, occurring primarily as alloys in the FeNi-metal phase; their abundance and distribution, however, also appear to depend, in part, on their volatility.

Larimer, John W.↗

NASA NTRS1988

Refractory lithophile elements

The abundance and distributions of refractory lithophile elements in chondritic meteorites and calcium-aluminum inclusions (CAIs) are presently noted to indicate that protoplanetary material was heated to very high temperatures. Three subsequent elemental fractionations have since been resolved: (1) that within the CAIs, indicating temperatures high enough to evaporate much of the refractory element content; (2) that of the refractory lithophile elements; and (3) that of Mg and Si, which indicates that the high temperatures were prevalent throughout the chondrite-formation region. Attention is given to models that have been proposed to explain elemental and isotopic abundances of refractory lithophile elements in chondritic material.

Larimer, John W.↗

NASA NTRS1988

Siderophile element fractionation

The cosmic volatility of the siderophile elements appears to have played an important role in the determination of their chondritic-material abundances, and has led to minor but significant fractionations: (1) the formation and fractionation of a component bearing elements more refractory than Fe, Ni, or Co; (2) the formation of a common FeNiCo component that separated from the common silicates; and (3) the depletion of moderately volatile elements by factors of as much as 5, where abundance decreased with increasing volatility. A refractory siderophile component is noted to be required in order to account for bulk siderophile trends among closely related chondrite groups.

Larimer, John W.↗

NASA NTRS1987

The trace element chemistry of CaS in enstatite chondrites and some implications regarding its origin

The trace element distribution in oldhamite (CaS) extracted from enstatite chondrites was determined by INAA. Prior to extraction, the petrologic setting of the grains was studied microscopically, and their minor element contents determined by microprobe analysis; samples that displayed a wide range of minor element contents were selected for detailed elementary analysis. Those samples of CaS suspected to be more primitive on the basis of their minor element and petrologic siting contain the entire inventory of the host meteorite's light REE (LREE) and Eu, plus 30-50 percent of the heavy-REE inventory. In less primitive samples, the LREE are less enriched although Eu remains highly concentrated. Several other elements, including lithophiles and chalcophiles, are most enriched in the most primitive CaS. It is suggested that oldhamite played a key role in the redistribution of these elements during the metamorphism and evolution of enstatite-rich material.

Larimer, John W.↗

NASA NTRS1986

Nebular chemistry and theories of lunar origin

The cosmic history of planetary matter is traced from nucleosynthesis through accretion in an attempt to understand the origin of the moon. It is noted that nebular processes must be considered in any theory of lunar origin and that planetary differentiation and volcanism determine the final character of lunar rocks. The moon's unique blend of nebular components suggests that the earth and moon accreted from the same mix of components as the proto-moon orbited the proto-earth, with the earth winning and the moon progressively losing, its solar complement of the components.

Larimer, John W.↗